1. Field of the Invention
The present invention relates to a power supply. More specifically, the present invention relates to a power supply for generating a voltage in accordance with an instruction signal.
2. Description of the Background Art
A general power supply must be designed such that an output voltage v.sub.0 stably and quickly follows an instruction voltage v.sub.r.
FIG. 10 is an electric circuit diagram of a switching regulator which is an example of such power supply. Referring to FIG. 10, a controller 1 receives an instruction voltage v.sub.r and an output voltage v.sub.0 which is fed back thereto. Controller 1 provides a control signal u, which is a difference between instruction voltage V.sub.r and output voltage v.sub.0, to a comparing input of a comparator 21 in a power converter 2. To a reference input of comparator 21, a triangular wave signal is applied from a triangular wave generator 22. Comparator 21 compares the control signal and the triangular wave signal, and applies a pulse signal having a pulse width corresponding to the level of the control signal u to a gate driving circuit 23.
Gate driving circuit 23 provides a driving pulse to the gate of a power MOS field effect transistor 24. Power MOS field effect transistor 24 has its drain connected to a positive pole of a DC power source through a primary coil of a transformer 25, while the source of power MOS field effect transistor 24 and the negative pole of DC power source 4 are grounded. Power MOS field effect transistor 24 turns on/off in accordance with a driving pulse applied to the gate, and provides an elevated (or lowered) pulse voltage to a secondary coil of transformer 25. The pulse voltage is made smooth by a filter 5 including a coil L and a capacitor C through rectifier diodes 26 and 27 to be a DC voltage, which DC voltage is supplied to a load 3.
The switching regulator shown in FIG. 10 can be represented by the block diagram of FIG. 11. Referring to FIG. 11, filter 5 and load 3 of FIG. 10 can be represented by the following equation (1). ##EQU2##
According to the equation (1), when .zeta.&lt;1, the denominator cannot be factored by a real number. In that case, there are portions where the gain becomes larger but the phase is delayed, as the frequency is increased in the characteristics of filter 5 and load 3, as shown in the bode diagram of FIG. 12.
In order to compensate for the instability caused by the fact that the phase is delayed while the gain is increased, it is necessary to provide a phase compensator so as to have the phase advanced as the gain is decreased.
FIG. 13 is a circuit diagram of a conventional switching regulator having a phase compensator. In the switching regulator shown in FIG. 13, output voltage v.sub.0 is fed back to an amplifier 7 through a phase compensating feedback element 6, the difference between the fed back voltage and the instruction voltage V.sub.r is amplified by amplifier 7, and the amplified output is compared with the triangular wave signal from triangular wave generator 22 in comparator 21. Except for this point, it is the same as the circuitry of FIG. 10. Phase compensating feedback element 6 includes a parallel circuit of a resistor R11 and a capacitor C11, and a grounded resistor R12, and the voltage at the node between resistor R12 and the parallel circuit including resistor R11 and capacitor C11 is input to amplifier 7.
Referring to FIG. 13, the transfer function when the output voltage v.sub.0 is viewed from the instruction voltage v.sub.r is represented by the following equation (2 ). ##EQU3##
Though the transfer function follows the instruction, voltage this relation is rather complicated.
The block diagram of the switching regulator shown in FIG. 13 is as shown in FIG. 14. Referring to FIG. 14, phase compensating feedback element 6 can be represented by the following equation (3): ##EQU4##
With the phase compensated feedback element 6 shown in FIG. 13, the phase cannot be advanced without increasing the gain, and therefore stable operation cannot be expected under the condition of .zeta.&lt;1.